1. Technical Field of the Invention
The present invention relates to a blade for a gas turbine, and more particularly to a gas turbine blade adapted to be internally cooled by a coolant steam flowing through the interior of the gas turbine blade.
2. Description of the Related Art
A conventional gas turbine for a combined cycle is shown in FIG. 5 to FIG. 7. As is shown in FIG. 5, the gas turbine 1 includes a plurality of alternately disposed stationary blades 2 and moving blades 4. A main gas flow of high temperature is fed to the gas turbine 1 from a combustor (not shown) to effect the moving blades 4 via the stationary blades 2 and rotate a rotor of the turbine at a high speed to thereby generate power.
Each of the stationary blades 2 of the turbine is provided with coolant steam pipes 3. Further, a plurality of radial coolant steam flow path sections a are formed inside the stationary blade 2, as can be seen in FIG. 7, wherein these coolant steam flow path sections are serially interconnected at a base end portion and a tip end portion of the stationary blade to thereby form a serpentine passage that extends from a leading edge side to a trailing edge side of the turbine stationary blade.
When the gas turbine is put into operation with the high temperature main gas being fed to the gas turbine, the stationary blade 2 is heated. In this case, however, the coolant steam is supplied to the coolant steam flow path section a formed inside of the stationary blade 2 of the turbine and located adjacent to the leading edge thereof to sequentially flow through the coolant steam flow path sections which form the serpentine passage. Hence, the stationary blade 2 of the turbine can be cooled from the internal surfaces thereof. Moreover, the coolant steam used for cooling the stationary blade 2 of the turbine is subsequently recovered through the coolant steam pipe 3 to be supplied to a succeeding system.
As can be seen from the above, in the combined cycle plant where the gas turbine is used in combination with a steam turbine, steam cooling has been proposed for cooling the gas turbine to enhance the thermal efficiency.
Next, a conventional air impingement cooling scheme is illustrated in FIG. 8 to FIG. 10. Inserts 5a, 5b and 5c are respectively disposed within radially partitioned compartments of a stationary blade 2 for a turbine, each insert having a predetermined gap relative to the inner wall surface of the compartment and a number of small holes 6 are formed therein. Additionally, a number of small holes 7 are also formed in the surface of the stationary blade 2 of the turbine in a predetermined pattern.
In the turbine stationary blade 2 of the structure mentioned above, the cooling air flowing into the inserts 5a, 5b and 5c is ejected through the individual small holes 6 formed in the inserts 5a, 5b and 5c to impinge on the inner wall surfaces of the stationary blade 2 of the turbine to thereby cool the stationary blade 2 from the inside (refer to FIG. 10). Subsequently, the cooling air is ejected from the small holes 7 formed in the surface of the stationary blade to film cool the latter.
When impingement cooling using the cooling air, the ratio t/c of blade thickness t to chord length c of the stationary blade 2 of the turbine, as shown in FIG. 7, is ordinarily selected so as to be at least 0.2 in order to facilitate insertion of the inserts 5a-5c into the interior of the stationary blade 2 of the turbine and ensure an uniform pressure distribution within the interiors of the individual inserts.
Nevertheless, a turbine stationary blade of a profile having the blade thickness/chord length ratio t/c mentioned above requires a large amount of coolant steam, e.g., a major proportion of the steam for the steam turbine of the combined cycle, because convection cooling must be adopted for steam cooling.
Furthermore, leakage of the coolant steam in the coolant steam piping will seriously affect the combined cycle (bottoming cycle), and possibly make the plant nonfunctional.
Also, the diameters of the pipes used in a complicated coolant steam piping system have to be increased due to the demand for a large amount of coolant steam, and this presents problems with respect to the cost and the space for installation of the coolant steam piping arrangement of individual pipes.